DESCRIPTION: Acute Respiratory Distress Syndrome (ARDS) results in significant mortality and morbidity in critically ill children. Despite aggressive treatment. approximately 45% of the 2,500 children afflicted with ARDS each year will die and 15% of survivors will suffer significant impairment in functional status. Prone positioning is a promising therapeutic adjunct to improve gas exchange and, when used early, may improve the clinical course of the patient's acute lung injury (ALI)/ARDS. The purpose of this project is to conduct a multicenter, randomized, non-crossover, controlled clinical trial comparing early, repeated and prolonged prone positioning with supine positioning in children with acute lung injury or acute respiratory distress syndrome. Subjects will include 180 ventilated children with ALI/ARDS. Subjects will be randomly assigned to prone or supine positioning during the acute phase of their ALI/ARDS illness for a maximum of 7 days. After study treatment (prone or supine positioning) all patients who are still intubated will be positioned supine (usual care) until they are successfully extubated or die. Other than positioning, subjects in both groups will be managed in exactly the same way during, the 28-day period. The primary endpoint is ventilator free days during the 28-day period. Secondary end points, include time to recovery of lung injury, organ failure free days, functional outcome, and mortality.